It is frequently convenient to carry something on one's back. This way it is possible to manage a fairly large and even heavy load while still leaving the hands free with the load set so that the person carrying it can walk naturally. This is done when camping to carry one's camping gear, in the military for transporting equipment such as radios, by school children who often carry their books in a pack, and in marching bands where members often have instruments, e.g. drums, carried on the back, and it is standard for guitars and similar instruments to be carried this way.
A typical backpack, the most common system employed for carrying something on the back, comprises a semirigid support, which may be formed as an open frame or an imperforate panel, having a front face and a back face. Straps attached to the support go over the user's shoulders and retain the support with its front face engaging the user's back and the back face turned rearward away from the user. A bag and various attachments are provided on the rear face for holding whatever is being transported. A similar system is used for carrying a bass drum in a marching band or a two-way radio on a military patrol.
A major issue with a backpack is comfort. The human back varies considerably from person to person, and two people of the same height and weight are likely to find different backpacks more comfortable than others. Hence it is standard to provide various elements of adjustable padding, numerous ways to adjust and position the shoulder straps, and so on in order that the backpack can be made comfortable for a given wearer.
All the known systems require that the user not only make various adjustments to the pack and its padding, but also that the user know what adjustments to make. The result is that only the most experienced users of backpacks have truly found something that is comfortable, and even so what is comfortable one day with a given load might be uncomfortable the next, or with a different load.
Another significant problem with backpacks, particularly encountered when used by children, is that the considerable load is applied to the user in such a manner as to be unhealthy. A young back can actually be injured by carrying a big pack that applies the weight in a single location, and in general the packs can rub in spots and become quite uncomfortable.